Nested SELECT FROM BSEG - performance

Hello!

I have a very simple program with 2 nested SELECTs, but with huge amount of data, and as the worst thing, on a cluster table BSEG with non-key access. I need to get the first set of data based on a certain criteria, and then the second set based on the first set. I can't use index tables such as BSID etc. Here is my program: